1. Field of the Invention
The present invention relates to an electrostatographic printing process for forming one or more colourless toner images in combination with at least one colour toner image and to an electrostatographic printer for carrying out said process.
2. Description of the Related Art
Electrostatographic printing operates according to the principles and embodiments of non-impact printing as described e.g. in "Principles of Non-impact Printing" by Jerome L. Johnson (1986)--Palatino Press--Irvine, Calif., 92715 U.S.A.
Electrostatographic printing includes electrographic printing in which an electrostatic charge is deposited image-wise on a dielectric recording member as well as electrophotographic printing in which an overall electrostatically charged photoconductive dielectric recording member is image-wise exposed to conductivity increasing radiation producing thereby a "direct" or "reversal" toner-developable charge pattern on said recording member. Magnetic brush development is suited for "direct" as well as "reversal" development. "Direct" development is a positive-positive development, and is suited for producing pictures and text.
"Reversal" development is a "positive-negative" o vice versa development process and is of particular interest when the exposure derives from an image in digital electrical form, wherein the electrical signals modulate a laser beam or the light output of light-emitting diodes (LEDs). It is advantageous with respect to a reduced load of the electrical signal modulated light source (laser or LEDs) to record graphic information (e.g. printed text) in such a way that the light information corresponds with the graphic characters so that by "reversal" development in the exposed area of a photoconductive recording layer, toner can be deposited to produce a positive reproduction of the electronically stored original. In high speed electrostatographic printing the exposure derives practically always from electronically stored, e.g. computer stored, information.
As used herein, the term "electrostatographic" also includes the direct image-wise application of electrostatic charges on an insulating support, for example by ionography.
In the electrophotographic art multi-colour printers are of two types. In multiple-pass single-station printers, image receiving material is passed repeatedly through a single image-producing station, each pass being used to print a different colour separation image. In single-pass multiple-station printers, image receiving material is passed once successively through a number of image-producing stations, each such station serving to print a different colour separation image. Multi-colour printers are known that produce a plurality of colour toner images on a photoconductive member, drum or endless belt, wherefrom the toner images are transferred directly onto printing stock material such as a paper sheet or paper web material, the latter after toner image formation being cut into the desired printing frames. In an alternative way toner images are subsequently transferred first to an insulating belt from distinct image-forming stations and are then transferred together to a receiving sheet or web.
Development of latent electrostatic images may proceed with various kind of toner broadly divided into two types: wet or dry type (ref. e.g. IEEE Transactions on Electronic Devices, Vol. ED-19, No. 4, April 1972 "Xerographic Development Processes: A Review" by Thomas L. Thourson, p. 495-511).
In dry development the application of dry toner powder to the substrate carrying the latent electrostatic image may be carried out by different methods known as, "cascade", "magnetic brush", "powder cloud", "impression" or "transfer" development also known as "touchdown" development described e.g. in said IEEE Transactions on Electronic Devices, Vol. ED-19, No. 4, April 1972, pp.495-511.
It is often desirable to provide at least some parts of the image with different optical characteristics. Thus, for example, it may be desired that graphic images, i.e. pictures, exhibit a glossy appearance, whereas normally it is preferred that printed text has a more matte finish, making it easier to read. When the printing stock, e.g. resin sheet material or resin-coated paper, has a high gloss it may be desirable to coat selected parts of the printing stock surface with a material having a matte finish before or after depositing colour toner thereon. It is further advantageous, when using certain printing stock, e.g. very smooth highly calendered paper, that has poor adherence to selected colour toners to pre-coat the surface of that printing material with colourless toner improving the adhesion (fixing) in the areas that have to receive the colour toners.
Furthermore, it may be desired to coat the coloured toner images with a layer that gives protection against degradation by mechanical contact (abrasion) and/or radiation such as ultraviolet radiation that may attack the colouring matter in the toner and give rise to colour-fading or discolouration.
So, all these measures follow the desire to apply in discretionary way colourless toner in direct contact with the printing stock surface and/or as overcoat on (an) already formed colour toner image(s) at some stage in the colour image formation.
It is known from published European patent application EP-A-0486235 (Konica Corporation) to adhere uniformly a transparent colourless toner layer onto a recording material carrying already (a) coloured toner image(s) and fixing said transparent toner layer together with the colour toner image(s) on the recording material. According to that technique the toner is applied non-image-wise which means in large quantity and follows the relief pattern of the already formed colour toner images. By said technique it is the intention to produce high glossiness over the whole surface of the toner receptor material.
It is further known from European patent specification EP-A-0081887 (Coulter Stork Patents B.V.) to form a protective transparent coating on a coloured toner image by depositing a transparent protective toner electrophotographically and in registration on said coloured toner image. For that purpose a multi-colour original from which the multi-colour reproduction is to be made is split up in different light-transmitting colour separation images. Such a method is limited to the application of the coating in only those parts corresponding with one or more of the separation images used in the exposure of the photoconductive recording member.